1. Field of the Invention
The present invention relates to a fuse blowout detector circuit which detects the blowout of a fuse. The detection is made independently of the ON/OFF state of switch in an electrical circuit, that includes the switch and the fuse and is located between a power supply and a load device.
2. Description of the Background Art
It is important to detect the blowout of a fuse in an electrical circuit protected by the fuse. For this purpose, a fuse with an alarm contact may be employed or a fuse blowout detector circuit may be provided across the fuse.
FIG. 5 shows a conventional circuit for detecting a fuse blowout, as disclosed in Japanese Published Patent Application 1-127157 of 1989. In the Figure, the numeral 100 indicates an external direct current power supply (hereinafter referred to as the "power supply"), 101 a load device driven from the power supply 100, 102 a fuse, and 103 a circuit for detecting a fuse blowout and OFF-condition of an external power supply. The circuit 103 comprises a photocoupler 104 for outputting a fuse blowout signal, a resistor 105 for restricting the forward current of a light-emitting diode 104A of the photocoupler 104, a pull-up resistor 106 for making the fuse blowout signal inactive, and a switching transistor 107 acting as switch. The load 101 and power supply 100 are located at a position external to the fuse circuit structure, as indicated by the dotted line, and are connected to the internal fuse structure via connectors A and B.
The operation of the circuit shown in FIG. 5 will now be described. When the fuse 102 is operative to pass current, no current flows in the fuse blowout detector circuit 103 and a phototransistor 104B of the photocoupler 104 is not switched ON. For this reason, a FUSEL signal, i.e. fuse blowout signal, is kept high by the pull-up resistor 106 and the FUSEL signal remains OFF.
When the fuse 102 is blown, a current flows in the light-emitting diode 104A of the photocoupler 104 in the fuse blowout detector circuit 103, switching ON the transistor 104B of the photocoupler 104. Hence, the FUSEL signal is switched low to turn ON the fuse blowout signal.
It should be noted that in this fuse blowout detector circuit 103, a fuse blowout can be detected only when the power supply 100 and the load device 101 are wired externally and the transistor 107 is ON. If the transistor 107 is not ON, the condition of the fuse cannot be checked.
FIG. 6 shows a second conventional circuit for detecting the blowout of a fuse provided with an alarm contact, as disclosed in Japanese Published Patent Application 1-124647 of 1989.
Referring to FIG. 6, external wiring is as shown in FIG. 5, and 106 indicates a pull-up resistor for making the fuse blowout signal inactive and 108 a fuse with an alarm contact.
The operation of the circuit shown in FIG. 6 will now be described. When the fuse with alarm contact 108 is operative to pass current, an alarm contact 108A built therein is OFF and the FUSEL signal is kept high by the pull-up resistor 106. As a result, the fuse blowout signal remains OFF. When the fuse 108 is blown, the built-in alarm contact 108A is designed to be switched ON, thereby switching the FUSEL signal low and turning ON the fuse blowout signal.
The conventional fuse blowout detector circuit constructed as described above has a disadvantage that it cannot detect the blowout of a fuse except when an external power supply and load device are connected to the fuse and a transistor serving as the switch of the load device is ON (closed). In other words, if the transistor is not ON, it cannot be checked whether the fuse is blown. As a result, it may be uncertain whether the indication of an OFF fuse blowout condition is due to the operability of the fuse or the failure of the transistor.
In addition, the fuse blowout detector circuit employing the fuse with an alarm contact has a disadvantage in that it is not suitable for being economical or compact because its fuse and fuse holder are high-priced and large-sized.